The central goal of this proposal is to define how syndecan interactions modulate lung injury responses in systemic inflammatory diseases. Dysregulated lung injury responses to systemic inflammatory diseases, such as sepsis, can lead to acute lung injury (ALI) and are a major cause of morbidity and mortality. However, many patients survive the initial phase of lung injury and do not progress to ALI or to the more severe form of ALI, acute respiratory distress syndrome (ARDS). These data suggest the importance of endogenous protective mechanisms that attenuate or reverse disease progression, but the underlying biology remains to be elucidated. Syndecans comprise a major family of cell surface heparan sulfate proteoglycans (HSPGs). Syndecans function as coreceptors on the cell surface and also as soluble HSPGs in the extracelular environment because its ectodomain can be shed under inflammatory conditions. Syndecans bind to ligands through its heparan sulfate (HS) chains, a glycosaminoglycan that binds to and regulates several inflammatory mediators implicated in lung injury. However, the precise role of syndecan interactions in inflammatory lung injury has yet to be determined. Syndecan-1 null mice show increased lung injury and mortality when subjected to animal models of systemic inflammatory diseases, such as endotoxic shock, Gram-positive toxic shock, or sepsis. Syndecan-1 shedding is induced in the lungs of wild type mice by the systemic inflammatory challenge, and inhibition of shedding exacerbates lung injury, whereas administration of purified syndecan-1 ectodomain or HS improves disease parameters. Based on these data, this proposal will examine the overall hypothesis that syndecan-1 modulates, in part, the highly complex mechanisms of lung injury and repair in systemic inflammatory diseases in 3 Specific Aims. Aim 1 wil define how syndecan-1 facilitates the resolution of lung inflammation. Aim 2 will determine how syndecan-1 attenuates lung injury and inflammation in sepsis. Aim 3 will establish that temporal syndecan-1 interactions regulate its shedding at the cell surface. These studies should define the key functions of syndecans in lung injury and repair, and provide a mechanistic foundation for the design and development of new therapeutic approaches against inflammatory lung diseases.